The present invention relates to a process for producing an electrode substrate, and more specifically, relates to a process for continuously producing an electrode substrate having uniform physical properties, particularly an electrode substrate for fuel cells at a low cost and a favorable productivity, and the electrode substrate obtained by the above-mentioned process.
Various processes have been proposed for producing an electrode substrate for fuel cells, etc. For instance, a process of subjecting dispersed carbon fibers to paper-manufacturing(refer to U.S. Pat. No. 3,998,689) and a process of chemical vapor depositing thermally decomposed carbon onto a web of carbon fibers(refer to U.S. Pat. No. 3,829,327) have been proposed.
There has also been proposed a process wherein an alcohol having a boiling point of higher than 150.degree. C. is used as a preparatory binder for forming a mat of pitch fibers and then the mat of pitch fibers is subjected to carbonization and heat-treatment in a non-oxidative atmosphere (refer to U.S. Pat. No. 3,991,169).
Other processes are known wherein a web comprising pitch fibers produced by blow-spinning are subjected to infusibilization and carbonization, thereby obtaining a web of carbon fibers(refer to U.S. Pat. No. 3,960,601).
A process for producing an electrode substrate for a fuel cell of monopolar type has been proposed, which comprises the steps of press-molding a mixture comprising short carbon fibers as the base, a carbonaceous resin binder such as a phenol resin and organic granules as the pore regulator, such as polyvinyl alcohol, polyethylene and polypropylene and calcining the thus press-molded body, has been proposed(refer to U.S. Pat. No. 4,506,028 and U.S. Pat. No. 4,666,755).
Although various electrode substrates have been produced by the above-mentioned processes, it is very difficult to have uniform physical properties all over the electrode substrate.
Namely, although the electrode substrate takes a thinplate form in general, the values of physical properties measured on the various points of the flat surface of the electrode substrate show fluctuation.
For instance, in the case where the compound containing the carbon fibers as the base material is supplied to a metal mold and is subjected to press-molding or rollmolding, the occurrence of an uneven supply of the compound is inevitable. As a result, the physical properties of the thus obtained electrode substrate are not uniform.
Particularly, in the case where the value of the bending strength of the electrode substrate fluctuates, there is a possibility that the electrode substrate may break during handling. And, in the case where the bulk density of the electrode substrate is uneven, portions of large thermal resistance and electric resistance occur. Furthermore, in the case where the value of thermal resistance fluctuates, portions of the substrate are locally heated to high temperatures to deleteriously accelerate the sintering of the catalyst thereby reducing the electrode's useful life.
Still more, in the case where the gas-permeability of the electrode substrate is uneven, since the resistance to the diffusion of reactant gas becomes uneven, there is a problem that the output specificity varies locally.
Since there is a limit in the productivity of electrode substrates by a non-continuous process (such as the above-mentioned press-process, it would be very desirable for a process to be provided which continuously produces electrode substrates at a favorable productivity.
As one such process for continuously producing electrode substrates, the extruding process is a possibility. However, the conventional compound for pressmolding, which comprises carbon fibers and a binder, exhibits poor fluidity and accordingly cannot be extruded.
In consideration of the above-mentioned situations, the present inventors have studied the process for continuously producing an electrode substrate of the uniform physical properties. As a result, it has been found that both the kneadability and the fluidity of a compound consisting of short carbon fibers and a binder are improved by mixing a molding additive with the compound. Accordingly, it is now possible with this invention to extrude the thus treated compound via extrusion processing technique. On the basis of their findings, the present inventors have arrived at the present invention.
Accordingly, one object of the present invention is to provide a process for continuously producing an electrode substrate having uniform physical properties at favorable production rates.
Further, another object of the present invention is to provide an electrode substrate which can be continuously produced and accordingly, can be produced at significantly reduced costs and which has uniform and favorable physical properties.